From 8dc1e5e4cd24e3e7ae1e7a8f52f1538e8a9d1ee3 Mon Sep 17 00:00:00 2001
From: David Philipson <david.philipson@gmail.com>
Date: Mon, 10 Feb 2020 10:55:52 -0800
Subject: [PATCH] Support type guards in filter-like functions

---
 src/chain.ts       | 19 +++++++++++++++++++
 src/reducers.ts    |  4 ++++
 src/transducers.ts | 13 ++++++++++++-
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/src/chain.ts b/src/chain.ts
index dfdffc9..5c8e0df 100644
--- a/src/chain.ts
+++ b/src/chain.ts
@@ -51,6 +51,7 @@ export interface TransformChain<T> {
     dedupe(): TransformChain<T>;
     drop(n: number): TransformChain<T>;
     dropWhile(pred: (item: T) => boolean): TransformChain<T>;
+    filter<U extends T>(pred: (item: T) => item is U): TransformChain<U>;
     filter(pred: (item: T) => boolean): TransformChain<T>;
     flatMap<U>(f: (item: T) => Iterable<U>): TransformChain<U>;
     flatten: T extends Iterable<infer U> ? () => TransformChain<U> : void;
@@ -59,10 +60,14 @@ export interface TransformChain<T> {
     mapIndexed<U>(f: (item: T, index: number) => U): TransformChain<U>;
     partitionAll(n: number): TransformChain<T[]>;
     partitionBy(pred: (item: T) => any): TransformChain<T[]>;
+    remove<U extends T>(
+        pred: (item: T) => item is U,
+    ): TransformChain<Exclude<T, U>>;
     remove(pred: (item: T) => boolean): TransformChain<T>;
     removeAbsent(): TransformChain<NonNullable<T>>;
     take(n: number): TransformChain<T>;
     takeNth(n: number): TransformChain<T>;
+    takeWhile<U extends T>(pred: (item: T) => item is U): TransformChain<U>;
     takeWhile(pred: (item: T) => boolean): TransformChain<T>;
 
     reduce<TResult>(
@@ -76,6 +81,7 @@ export interface TransformChain<T> {
     average: T extends number ? () => number | null : void;
     count(): number;
     every(pred: (item: T) => boolean): boolean;
+    find<U extends T>(pred: (item: T) => item is U): U | null;
     find(pred: (item: T) => boolean): T | null;
     first(): T | null;
     forEach(f: (item: T) => void): void;
@@ -120,6 +126,9 @@ export interface TransducerBuilder<TBase, T> {
     dedupe(): TransducerBuilder<TBase, T>;
     drop(n: number): TransducerBuilder<TBase, T>;
     dropWhile(pred: (item: T) => boolean): TransducerBuilder<TBase, T>;
+    filter<U extends T>(
+        pred: (item: T) => item is U,
+    ): TransducerBuilder<TBase, U>;
     filter(pred: (item: T) => boolean): TransducerBuilder<TBase, T>;
     flatMap<U>(f: (item: T) => Iterable<U>): TransducerBuilder<TBase, U>;
     flatten: T extends Iterable<infer U>
@@ -132,10 +141,16 @@ export interface TransducerBuilder<TBase, T> {
     ): TransducerBuilder<TBase, U>;
     partitionAll(n: number): TransducerBuilder<TBase, T[]>;
     partitionBy(pred: (item: T) => boolean): TransducerBuilder<TBase, T[]>;
+    remove<U extends T>(
+        pred: (item: T) => item is U,
+    ): TransducerBuilder<TBase, Exclude<T, U>>;
     remove(pred: (item: T) => boolean): TransducerBuilder<TBase, T>;
     removeAbsent(): TransducerBuilder<TBase, NonNullable<T>>;
     take(n: number): TransducerBuilder<TBase, T>;
     takeNth(n: number): TransducerBuilder<TBase, T>;
+    takeWhile<U extends T>(
+        pred: (item: T) => item is U,
+    ): TransducerBuilder<TBase, U>;
     takeWhile(pred: (item: T) => boolean): TransducerBuilder<TBase, T>;
 
     build(): Transducer<TBase, T>;
@@ -222,6 +237,10 @@ class TransducerChain<TBase, T> implements CombinedBuilder<TBase, T> {
         return this.compose(partitionBy(f));
     }
 
+    public remove<U extends T>(
+        pred: (item: T) => item is U,
+    ): CombinedBuilder<TBase, Exclude<T, U>>;
+    public remove(pred: (item: T) => boolean): CombinedBuilder<TBase, T>;
     public remove(pred: (item: T) => boolean): CombinedBuilder<TBase, T> {
         return this.compose(remove(pred));
     }
diff --git a/src/reducers.ts b/src/reducers.ts
index 6008289..46ca554 100644
--- a/src/reducers.ts
+++ b/src/reducers.ts
@@ -25,6 +25,10 @@ export function every<T>(pred: (item: T) => boolean): Transformer<boolean, T> {
     return remove(pred)(isEmpty());
 }
 
+export function find<T, U extends T>(
+    pred: (item: T) => item is U,
+): Transformer<U | null, T>;
+export function find<T>(pred: (item: T) => boolean): Transformer<T | null, T>;
 export function find<T>(pred: (item: T) => boolean): Transformer<T | null, T> {
     return filter(pred)(first());
 }
diff --git a/src/transducers.ts b/src/transducers.ts
index 76b4302..781cda3 100644
--- a/src/transducers.ts
+++ b/src/transducers.ts
@@ -172,7 +172,10 @@ class Filter<TResult, TCompleteResult, TInput>
         return this.pred(input) ? this.xf[STEP](result, input) : result;
     }
 }
-
+export function filter<T, U extends T>(
+    pred: (item: T) => item is U,
+): Transducer<T, U>;
+export function filter<T>(pred: (item: T) => boolean): Transducer<T, T>;
 export function filter<T>(pred: (item: T) => boolean): Transducer<T, T> {
     return xf => new Filter(xf, pred);
 }
@@ -473,6 +476,10 @@ class Take<TResult, TCompleteResult, TInput>
     }
 }
 
+export function remove<T, U extends T>(
+    pred: (item: T) => item is U,
+): Transducer<T, Exclude<T, U>>;
+export function remove<T>(pred: (item: T) => boolean): Transducer<T, T>;
 export function remove<T>(pred: (item: T) => boolean): Transducer<T, T> {
     return filter(item => !pred(item));
 }
@@ -550,6 +557,10 @@ class TakeWhile<TResult, TCompleteResult, TInput>
     }
 }
 
+export function takeWhile<T, U extends T>(
+    pred: (item: T) => item is U,
+): Transducer<T, U>;
+export function takeWhile<T>(pred: (item: T) => boolean): Transducer<T, T>;
 export function takeWhile<T>(pred: (item: T) => boolean): Transducer<T, T> {
     return xf => new TakeWhile(xf, pred);
 }